Pseudogap critical point inside the superconducting phase of cuprates

Recent high-field measurements of electrical transport in cuprates have revealed a new signature of the critical point p* where the pseudogap phase ends at T=0 in the absence of superconductivity: the carrier density drops abruptly from n=1+p above p* to n=p below [1,2,3]. Two questions arise: 1) Is the ground state of the pseudogap phase truly a metal, given the large upturn in the resistivity as low T, dubbed a “metal-to-insulator” crossover? 2) Is p* present inside the superconducting phase in zero field? Here we report low-temperature measurements of thermal conductivity in the cuprate superconductor Nd-LSCO for dopings across p*. We obtain the residual linear term at T=0, $\kappa_{0}/T$, as a function of magnetic field and doping. At high fields, we find that $\kappa_{0}/T$ satisfies the Wiedemann-Franz law at all dopings. This shows that the ground state of the pseudogap phase is a metal, and p* corresponds to a metal-to-metal transition at T=0. In zero field, a very similar drop in $\kappa_{0}/T$ across p* is observed, showing that p* is present in zero field inside the superconducting phase.\\\\~[1] S.~Badoux~{\it et al.}, Nature~{\bf 531},~210~(2016)\\~[2] F.~Lalibert\'e~{\it et al.}, arXiv:1606.04491~(2016)\\~[3] C.~Collignon~{\it et al.}, arXiv:1607.05693~(2016)